1. Field of the Invention
The present invention relates to a device generating wavelength dispersion, and a device used to compensate for wavelength dispersion accumulated in an optical fiber transmission line.
To be more specific, the present invention relates to a device which uses a virtually imaged phased array in order to generate wavelength dispersion.
2. Description of the Related Art
A transmitter for transmitting information with light in a conventional optical fiber communications system transmits optical pulses to an optical fiber. Light from the optical fiber is received by a light receiver.
However, the optical fiber has wavelength dispersion, which is also called chromatic dispersion.
The wavelength dispersion changes the pulse width of a signal of the system, so that the quality of the signal is degraded.
To be more specific, the propagation speed of signal light within an optical fiber depends on the wavelength of the signal light because of the wavelength dispersion.
For example, when an optical pulse having a long wavelength (such as an optical pulse having a wavelength of red color) propagates faster than an optical pulse having a short wavelength (such as an optical pulse having a wavelength of blue color), this dispersion is called normal dispersion.
Inversely, an optical pulse having a short wavelength (such as a blue pulse) propagates faster than a pulse having a long wavelength (such as a red color pulse), this dispersion is called abnormal dispersion.
Accordingly, if signal light pulses include red and blue pulses when being transmitted from a transmitter, they are separated into red and blue pulses while propagating within an optical fiber, and the respective optical pulses are received by a light receiver at different times.
Another example of an optical pulse transmission is such that: when signal light pulses having wavelength components which are successive from blue to red are transmitted, the components respectively propagate within an optical fiber at different speeds. Therefore, the time width of the signal light pulses is widened within the optical fiber, which causes distortion. Since all of pulses include a component within a finite wavelength range, such wavelength dispersion is very common in an optical fiber communications system.
Accordingly, especially in a high-speed optical fiber communications system, it becomes necessary to compensate for wavelength dispersion so as to obtain a high transmission ability.
To compensate for such wavelength dispersion, an optical fiber communications system requires a reverse dispersion component which gives to an optical pulse wavelength dispersion reverse to that occurring in an optical fiber.
As one reverse dispersion component, a device including a virtually imaged phase array, namely, a portion called VIPA 1 shown in FIG. 1 is conventionally proposed by Japanese Patent Application Nos.10-534450 and 11-513133.
The VIPA plate 1 generates light which propagates from the VIPA plate 1 with angular dispersion. This device also includes a light returning device 2 for returning light to the VIPA plate 1, and for causing multiple reflection within the VIPA plate 1.
The above described device is implemented by comprising a device including the VIPA plate 1, which receives input light of a wavelength within a continuous wavelength range, and successively generates corresponding output light. The output light can be distinguished spatially from output light having another wavelength within the continuous wavelength range (for example, the output light proceeds in a different direction). If this output light can be distinguished with a proceeding angle, this device is proved to have angular dispersion.
The VIPA comprises a transparent area and a transparent member.
Light passes through the transparent area, so that it can be input/output to/from the VIPA.
The transparent member 3 has first and second surfaces.
The first and the second surfaces are reflection planes. The reflection plane of the second surface has both a characteristic which reflects light, and a characteristic which passes part of input light.
Input light passes through the transparent area, Is received by the VIPA plate 1, and reflected many times between the first and the second surfaces of the transparent member, so that a plurality of lights pass through the second surface.
The plurality of passed lights interfere with one another, thereby generating output light 4.
The input light has a wavelength within a continuous wavelength range, and the output light can be distinguished spatially from light having another wavelength within the wavelength range.
The light returning device 2 can return the output light to the second surface in exactly the reverse direction. The output light passes through the second surface, is input to the VIPA plate 1, and multiple-reflected within the VIPA plate 1, so that the output light is output from the transparent region of the VIPA plate 1 to an input path.
Additionally, the above described device generates a plurality of output lights that have the same wavelength as that of the input light, and have different interference orders.
This device also comprises a light returning device 2 which returns output light corresponding to one interference order to the VIPA plate 1, and does not return the other output lights.
As a result, only light corresponding to one interference order is returned to the VIPA plate 1.
Furthermore, the above described device comprises the VIPA plate 1, the light returning device 2, and a lens 5.
The VIPA plate 1 receives input light, and generates corresponding output light which propagates from the VIPA plate 1.
The light returning device 2 receives the output light from the VIPA plate 1, and returns the output light to the VIPA plate 1.
The lens 5 is positioned so that (a) the output light proceeds from the VIPA plate 1 to the light returning device 2 via the lens 5 by being focused on the light returning device 2 by the lens 5, (b) the output light is returned from the light returning device 2 to the VIPA plate 1 via the lens 5 by being directed toward the VIPA plate 1 by the lens 5, and (c) the output light proceeds from the VIPA plate 1 to the lens 5 in parallel and in the direction reverse to the output light which is returned from the lens 5 to the VIPA plate 1.
Additionally, the output light which proceeds from the VIPA plate 1 to the lens 5 does not overlap the output light which is returned from the lens 5 to the VIPA plate 1.
Furthermore, the above described device comprises a device comprising a mirror 6.
The VIPA plate 1 receives input light, and generates corresponding output light which propagates from the VIPA plate 1.
The lens 5 focuses the output light on the mirror 6, which reflects the output light, so that the reflected light is returned to the VIPA plate 1 by the lens 5.
The mirror 6 is formed so that the device makes constant wavelength dispersion.
As described above, the VIPA plate 1 has an angular dispersion function like a diffraction grating, and enables wavelength dispersion compensation. Especially the VIPA plate 1 has a characteristic of having large angular dispersion, and can easily provide a practical reverse dispersion component.
However, the device that uses the VIPA plate for wavelength dispersion compensation still has a problem that the characteristic of transmittance of a wavelength is not flat, and becomes a periodical characteristic which is asymmetric with respect to the peak of a wavelength within each transparent bandwidth as shown in FIG. 2.
If a device having such a transparent characteristic which is not flat and asymmetric exists on an optical transmission line, distortion occurs in a signal light pulse waveform transmitted from a transmitter, and the signal cannot be properly transmitted. Especially, if wavelength dispersion compensation is made by adopting a device using a VIPA plate in many stages in a long-haul optical fiber communications system for which large wavelength dispersion must be compensated, the above described undesirable transparency characteristics is superposed. As a result, a signal light pulse is significantly degraded.
Accordingly, it is desirable that a device using a VIPA plate has a flat wavelength characteristic of output light.
The above described transparency characteristic which is not flat and asymmetric occurs in principle in a device using a VIPA plate due to the following reason.
Light having wavelengths, which is output from the VIPA plate, has a plurality of proceeding directions of different interference orders. Therefore, the intensity of the light is dispersed into lights having the plurality of interference orders.
In the device using the VIPA plate, unnecessary interference order light must be cut, and only one necessary interference order light must be extracted. Accordingly, if unnecessary interference order light to be cut is output from the VIPA plate, optical loss corresponding to this light occurs.
In the meantime, whether or not each interference order light is output from the VIPA plate depends on whether or not the direction where each interference order light is to proceed is included among the proceeding directions of parallel light components within the input light which is focused on the VIPA plate by the lens.
Accordingly, the degree of occurrence of unnecessary interference order light differs depending on a wavelength.
A direction which satisfies an interference condition on a short wavelength side is a direction upward from a central wavelength, whereas a direction which satisfies an interference condition on a long wavelength side is a direction downward from the central wavelength.
At this time, interference light does not occur unless a region where optical energy exists and a region which satisfies an interference condition overlap. However, since an angle between different orders is large under the interference condition on the long wavelength side, a plurality of interference lights do not occur. Therefore, a lot of energy is distributed also to a portion of an unnecessary order, which produces dropped light. As a result, an optical loss on the long wavelength side increases.
Accordingly, losses on the short and the long wavelength sides get out of balance, so that asymmetry occurs in the transparency characteristic.
A method improving such a transparency characteristic, and implementing a desirably flat transparency characteristic in a device using a VIPA is not specifically disclosed by Japanese Patent Application Nos. 10-534450 and 11-513133.
In the meantime, as a conventional method preventing the degradation of an optical signal due to the transparency characteristic of an optical device, a method comprising an optical compensation filter having a transparency characteristic, which converts a transparency characteristic into a flat characteristic, is proposed by Japanese Patent publication No. 11-72756.
However, the transparency characteristic of an optical filter which adopts conventionally known Mach-Zehnder Interference using a waveguide, or Fabry-Perot interference using an etalon becomes a periodical characteristic which is symmetrical with reference to the peak of a wavelength as shown in FIG. 3. Therefore, strict flattening is impossible for a device using a VIPA plate.
Note that an asymmetric filter can be implemented by overlaying Fourier filters whose passing light cycles are different in many stages. However, this implementation is impractical due to the following reasons. As the number of filters increases, so does the loss of passing light. Additionally, the filters cost high.